1. Field of the Invention
The present invention relates to a vertical heat-treatment apparatus and more particularly to a vertical heat-treatment apparatus having a boat transfer mechanism for transferring boats, each of which carries wafers, between a wafer transfer position and a boat load/unload position.
2. Description of the Related Art
As unmanned control system and automation system have recently developed in a clean room, there has been visualized a system for transferring wafer carriers (cassettes) by means of pilotless transferring robots between machines for manufacturing semiconductor devices. For this purpose, a loading/unloading port for automatically transferring wafer carriers is provided in each manufacturing machine.
It is required that the space of the clean room be as small as possible in order to exhaust dust in the clean room in a short time so as to maintain a high degree of cleanness. Since a vertical heat-treatment furnace has a smaller floor space than a horizontal heat-treatment furnace, the employment of the vertical heat-treatment furnace allows for effective usage of the floor space. Further, the vertical heat-treatment furnace has an advantage that a wafer boat is transferred without contacting the inner wall of a reaction tube.
In the vertical heat-treatment furnace is provided a wafer transfer device for transferring semiconductor wafers from the carriers to the boat.
U.S. Pat. No. 4,770, 590 discloses a wafer transfer mechanism for a vertical heat-treatment furnace. However, this wafer transfer mechanism is of a semi-automatic type so that an operator must manually set carriers on the station. At the time of this setting, undesirable dust enters the furnace and semiconductor wafers are contaminated, as a result increasing the fraction defective of the semiconductor devices.
Japanese Published Unexamined Patent Application No. 61-144821 discloses an automatic wafer transfer mechanism which automatically transfers wafers. With this mechanism, a transfer robot sets a single carrier on a station and wafers are transferred by means of a transfer device from the carrier to a boat. Since, after the transfer has been completed, the transfer robot must return the empty carrier to the station and set a new single carriage on the station, the transfer requires much time. Specifically, it takes about three minutes to transfer wafers from a carrier (accommodated twenty five wafers) to the boat. Therefore, for example, when wafers are transferred from six to eight carriers to the boat, it will take about twenty minutes to complete the transfer of the wafers to the boat. Since the transfer mechanism has a separate transfer robot, the mechanism has the demerit that it requires a large floor space in total. Further, the robot is expensive and its complicated structure leads to a lot of cost and time to maintain and check it.